European patent No. 0133703 entitled "Centre Satellite Numerique de Raccordement d'abonnes" (Digital Satellite Center for Connecting Subscribers) describes a connection unit that may be local or remote. In addition, this "versatile" connection unit makes it possible to combine both analog and digital subscribers on concentrators connected to a digital control unit by digital multiplex lines, said concentrators being local or remote.
Such connection units can be used for connecting metal wire subscribers only, i.e. either analog subscribers or 144 Kbit/s digital subscribers using two 64 Kbit/s channels in circuit mode and one 16 Kbit/s channel in frame mode, known to the person skilled in the art as narrow band ISDN 2B+D lines. In addition, the multiplex lines between concentrators and the digital control unit and also between the digital connection unit and the parent center convey all information, i.e. speech, data, and signalling, in synchronous multiplexes, e.g. at 2.408 Mbit/s having 125-microsecond frames with 32 time slots each.
Unfortunately, subscriber needs with new services, such as video services and data services at very high data rates, for example, can no longer be satisfied by the capacities of present subscriber lines, whereas optical fibers are capable of digital data rates that may be as high as 600 Mbit/s or even more.
Asynchronous time division (ATD) makes it possible to multiplex and switch all types of information (speech, images, hi-fi sound, data any data rate, signalling, etc. . . . ) in a single form of information packet "cell", with cells being fixed in length, and comprising, for example, a 4-byte header portion and a 32-byte information portion. Standardization relating to ATD is currently being performed by the CCITT. Examples of the contents of a header and descriptions of asynchronous time division can be found CCITT recommendation I.121 at paragraphs 3,4.3, et seq, and examples of asynchronous time switching matrices can be found in: ISS 84--Florence, 7-11 May 1984--Session 32C, paper 2, Asynchronous Time-Division Techniques: An experimental packet network integrating video-communications, by A. Thomas, J. P. Coudreuse, and M. Servel; and in ISS 1987--Phoenix, B5 pages 567 to 372 Switching techniques for asynchronous time division multiplexing (or fast package switching) by M. Dieudonne and M. Quinquis.
The object of the invention is to connect different types of subscriber such as analog and digital subscribers on metal wires asynchronous time division (ATD) digital subscribers on metal wires and on optical fibers, and also synchronous subsystems such as private exchanges (PABX) having 2 Mbit/s accesses, for example, and synchronous digital satellite centers, e.g. of the type described in above-mentioned European patent No. 0133703.